HELP!!!

I am making a pot roast for the first time today, I put a 3lbs roast in, then water and spices, celery, onion, carrots and potatos in that order. I got up a second ago to get a refill of my coffee and the potatos are spotted black!! It looks like mold to me or something but they were perfectly fine when I peeled and cut them up this morning. What is this? Is the whole roast bad?